Engine exhaust gases contain a number of gases and particulates that it is undesirable to release to the atmosphere. The gases typically include hydrocarbons, carbon monoxide (CO), carbon dioxide (CO2) and nitrogen oxides (NOx) whilst the particulates include carbon particles and other solid and liquid phase components.
It is known to provide engine aftertreatment apparatus employing a diesel oxidation catalyst (DOC) module, a diesel particulate filter (DPF) module and a selective catalytic reduction (SCR) catalyst module. FIG. 1(a) shows an example of such an apparatus 100.
The apparatus 100 has a DOC module 120 coupled to an exhaust outlet 111 of an engine 110. The DOC module 120 is provided upstream of a DPF module 130 which is in turn provided upstream of an SCR module 140. An ammonia source 142 is arranged to inject ammonia gas or an ammonia precursor compound into a flowstream of exhaust gases from the DPF module 130 to the SCR module 140. In some systems, the order of the SCR catalyst and DPF can be reversed.
The DOC module 120 is arranged to oxidise hydrocarbons and CO to form CO2 and H2O. Some NO2 may also be formed by oxidation of NO in the exhaust gas stream. The generation of NO2 by the DOC 120 can be helpful in promoting formation of N2 in the SCR module, and in oxidising particulate matter trapped on the DPF. The DOC module 120 may comprise a ceramic monolith having an oxidation catalyst provided thereover. The catalyst may be applied by means of a wash coat, and may comprise one or more types of metal such as platinum, rhodium, palladium or an alloy of two or more metals.
The DPF module 130 is a wall-flow filter module and contains a porous DPF filter material arranged to trap soot particles entrained in the exhaust gases. The filter material is coated with an oxidation catalyst that oxidises the trapped particles to form CO2. DPF modules 130 are typically arranged to operate in a passive regeneration role in which oxidation of the trapped particles takes place to a sufficient extent without a requirement to provide external heat energy to the DPF module 130 other than that provided by the exhaust gases as exhausted from the engine 110 at the exhaust outlet 111.
FIG. 2(a) is a cross-sectional schematic illustration of a wall-flow filter 30 in which exhaust gases flow through parallel channels 31C defined by walls 31 of the filter 30. The channels 31C are blocked alternately at a first end 30A or a second end 30B such that gases entering the filter 30 through one channel 31C at the first end 30A are forced to flow through a wall 31 of the filter 30 in order to exit the filter through an adjacent channel 31C at the second end 30B.
FIG. 2(b) is a cross-sectional schematic illustration of a filter 40 of flow-through or deep bed type. The filter has a porous matrix 41 having a plurality of pores or channels 41C therethrough. Gases entering the filter 40 at a first end 40A may pass through the filter 40 by passing along the channels 41C and emerge from a second end 40B opposite the first end 40A.
The problem exists that the amount of soot trapped in the DPF module 130 may become too high, for example due to the DPF module 130 not attaining a sufficiently high temperature for a sufficiently long period during a given drivecycle. An active regeneration operation may therefore be required. A number of technologies exist for performing active regeneration operations. One common technology is an operation in which the engine is operated under conditions in which additional fuel is injected into the cylinder late in the engine operating cycle. Excess unburned fuel becomes entrained in the exhaust gas and enters the DOC module 120. The DOC module 120 oxidises the unburned fuel generating heat which causes an increase in the temperature of catalyst and soot in the DPF module 130, promoting oxidation of soot particles trapped in the DPF module 130.
It is to be understood that it is undesirable to operate the engine 110 under active regeneration conditions for at least the following reasons: (1) the process causes an increase in fuel consumption and CO2 and NOx emissions; and (2) some of the fuel mixes with engine oil causing a reduction in viscosity and lubricity of the oil.
Thus it is desirable to keep a flow path of exhaust gases from the engine 110 to the DOC module 120 and DPF module 130 as short as possible, so that active regenerations may be performed in as short a time period as possible and passive regeneration can be maximised.
The SCR module 140 is arranged to reduce NOx gases passing therethrough to N2. A reduction catalyst such as Cu, Fe, V or a mixture thereof is provided in the SCR module 140 and ammonia (NH3) is injected into the flow of exhaust gases into the SCR module 140, either directly or as an ammonia precursor, in order to effect reduction of the NOx gases.
Exhaust gases flowing through the SCR module 140 are exhausted from the apparatus 100 by means of an exhaust outlet 150 downstream of the SCR module 140.
In some known arrangements, the total volume and weight of catalyst in the aftertreatment apparatus is reduced by combining the DPF and SCR modules in a single wall-flow filter. This is achieved by coating a DPF filter material with SCR catalyst (a reducing catalyst) rather than the usual DPF catalyst (an oxidising catalyst) to form a so-called SCR coated DPF (or SCRDPF) module.
FIG. 1(b) shows an example of an apparatus 200 having such an arrangement. Like features of the arrangement of FIG. 1(b) to that of FIG. 1(a) are provided with like reference numerals prefixed numeral 2 instead of numeral 1.
A conventional DOC module 220 is provided immediately downstream of an exhaust gas outlet 211 of an engine 210. An SCR coated DPF (SCRDPF) module 240 is provided downstream of the DOC module 220. An ammonia source 242 is arranged to inject ammonia gas or an ammonia precursor compound into a flowstream of exhaust gases from the DOC module 220 to the SCRDPF module 240. Exhaust gases flowing through the SCRDPF module 240 are exhausted from the apparatus 200 by means of an exhaust outlet 250 downstream of the module 240.
Because the DPF of the module 240 is coated with SCR catalyst and not the normal oxidation catalyst, the only way to oxidise trapped soot particles is by operating the SCRDPF module 240 at a higher temperature than would otherwise be desirable. However, operation of the module 240 at a higher temperature has the disadvantage that the NH3 that is provided in order to promote reduction of NO and NO2 to N2 in the presence of the SCR catalyst becomes oxidised and pre-stored NH3 is released and therefore does not reduce NO and NO2 to N2.
A further disadvantage of the SCRDPF module 240 is that it requires a relatively large package size and may need to be mounted underneath the vehicle. This results in an increase in a length of the flowpath of exhaust gases from the engine 210, and therefore a greater drop in temperature of the gases by the time they reach the module 240. Consequently active regeneration of the SCRDPF module 240 is required to be performed regularly in order to oxidise the soot particles trapped by it.
In addition to the disadvantage stated above that SCRDPF filters require frequent active regeneration events, such filters also suffer the disadvantage that they may provide an increased backpressure on exhaust gases flowing through the aftertreatment apparatus. This reduces an efficiency of the engine.
It is desirable to provide an improved engine aftertreatment apparatus that does not suffer the disadvantages of known SCRDPF modules 240.